1. Field of the Invention
This invention is concerned with a process for the preparation of particle board and is more particularly concerned with an improved process in which the metal plates, which come into contact with the particle boards during the forming process using an organic polyisocyanate as the binder, are fabricated from certain metals.
2. Description of the Prior Art
The use of organic polyisocyanates, alone or in admixture with an internal release agent, as the binder composition in the preparation of particle board is now well established in the art. U.S. Pat. No. 4,257,995/6, 4,258,169, and 4,352,696 describe the use of certain organic phosphates, thiophosphates, pyrophosphates and related phosphorus-containing compounds as internal release agents in such binder compositions. German Offenlegungsschrift 2,921,689 and 2,921,726 show the use of organic sulfonic acids and derivatives as internal release agents in similar organic polyisocyanate binder compositions. Processes have also been described in which the polyisocyanate is used as the binder composition and release agents are applied directly to the press plates utilized in preparing the particle boards; see, for example, U.S. Pat. No. 4,110,397 which shows applying a metallic soap to the surface of the caul plates prior to pressing.
The above types of binder compositions, comprising an organic polyisocyanate and, optionally, an internal release agent, are generally applied to the particles employed to form the particle board, either neat or in the form of an aqueous emulsion, prior to forming the particle board from the treated particles using heat and pressure. The organic polyisocyanate, and the internal release agent if one is used, can be admixed prior to application to the particles or, in a less preferred mode of operation, the two components of the binder composition can be applied separately to the particles.
After being treated with the binder composition, the particles (e.g. wood chips and other cellulosic or non-cellulosic material which is capable of being compressed and bound into the form of a board as discussed more fully hereafter) are then molded as boards using the action of heat and pressure. The necessary heat and pressure is generally applied utilizing a heavy duty press with heated metal platens. However, other forms of heat such as radio frequency radiation can be used. In many instances caul plates are employed and these are interposed between the platens of the press and the boards. In continuous operations, continuous belts may be used in place of individual caul plates. The caul plates or continuous belts are generally fabricated from aluminum, cold-rolled steel, hot rolled steel, or stainless steel. The press platens themselves are usually hot rolled steel.
Using the above compositions containing the aforesaid mold release agents or using polyisocyanate binders in combination with external release agents, it is found that the particle board, after being formed, will release readily from the surface of the caul plates, or from the platens if no caul plates are employed, without the need to apply any manual or mechanical force to effect separation of the board and caul plates or platens. Further, it is found that such release generally occurs readily throughout a prolonged production run of such particle boards involving a pressing cycle of approximately 5 minutes per board of 1/2" thickness. However, it is sometimes found, particularly in the case of caul plates or platens which have been fabricated from steel, that a deposit of binder composition can build up on the surface of the caul plates or platens, particularly on the periphery immediately surrounding the outer edges of the area of the metal surface which comes into contact with the particle board. This deposit of material results from slight leakage of binder composition from the mat during the pressing of the particle board. After prolonged operation of the caul plates or platens this buildup of residue, which residue tends to become degraded by heat as the production run progresses, can interfere with the ease with which the particle boards will release from the metal surface during the removal operation.
It has also been found that the ease with which the particle boards will release from the caul plates or platens when using polyisocyanate binders in association with internal or external release agents can vary to a significant extent according to the nature of the particles being used. Illustratively, boards which are prepared using some hardwood chips such as those derived from oak show a significantly greater tendency to adhere to the platens than do boards prepared from other chips such as those derived from pine and the like cellulosic materials.
We have now found that the ease of release of particle boards from the surface of the caul plates or platens which come into contact with the boards during the forming process, can be enhanced significantly by fabricating said plates or platens, or at least the surfaces thereof which come into contact with the boards, from magnesium or zinc. This finding not only helps to obviate the occasional difficulties described above, but can, in many cases, enable the level of release agent incorporated into the binder composition to be significantly reduced.